Laser ablation techniques are known for use in preparing thin films. Generally, a substrate and a target are arranged in a film-forming chamber which is evacuated to a high vacuum and is supplied with a desired ambient gas. Laser beam is emitted from a laser apparatus provided outside the film-forming chamber and is guided by an optical means to ablate the target. Lasers commonly used in these methods are capable of emitting pulsed beam having high peak power, examples Of which include CO.sub.2 lasers and excimer lasers. These lasers provide incident laser beam energy on the target surface typically in the range of 1-2 J/cm.sup.2 per pulse and on the order of 3.5 J/cm.sup.2 per pulse at maximum.
These conventional laser ablation methods for preparing thin films are advantageous in providing easy compositional control of the formed thin films and in providing fast film formation. Additionally, laser ablation does not require the use of an electromagnetic field and therefore is suitable for preparing thin films of high quality. However, these methods of laser ablation are disadvantageous for forming thin films because a part of the target surface which is ablated with laser beam is melted, whereby the composition in the ablated area is changed. That is the energy of the incident laser beam is partially absorbed as thermal energy by the target to increase its temperature and change the composition in the ablated area. As a result, the composition of the thin film which is being deposited varies in the direction of the thickness of the film. Accordingly, it has been difficult to prepare homogeneous superconducting oxide thin films using conventional laser ablation methods.
Thus, a need exists for an improved process for laser ablation for forming high-quality thin films, and particularly high-quality superconducting oxide thin films.